1. Field of the Invention
The present invention relates to an electrophotographic carrier for use in a developer for electrophotographic image forming apparatus. In addition, the present invention relates to a developer including the carrier, a developer container, and a process cartridge, an image forming apparatus and an image forming method using the developer.
2. Discussion of the Background
In electrophotography, an image is formed as follows:    (1) an electrostatic latent image is formed on an image bearing member such as a photoreceptor;    (2) a toner image is formed by adhering a charged toner to the electrostatic latent image; and    (3) the toner image is transferred onto a recording material such as a paper, and fixed.
Recently, the technology of color copiers and color printers using electrophotography which can produce documents including color images has been improved rapidly and broadly used. A full color image is formed by overlaying three primary color toner images (i.e., yellow, magenta and cyan toner images) and a black toner image. To obtain brilliant full color images having good color reproducibility, the surface of the toner image must be smoothened to some extent, to reduce light scattering. Therefore, conventional full color machines generally produce images having a glossiness of from 10 to 50% (i.e., from middle glossiness to high glossiness).
In particular, when a dry toner image is fixed to a recording material, contact heat fixing methods are typically used. In contact heat fixing methods, a toner image formed on a recording material is contacted and heated with a roller or a belt having a smooth surface, upon application of pressure to the toner image, resulting in fixation of the toner image on the recording material. The contact heat fixing methods have an advantage of having excellent heat efficiency in fusing a toner image. Therefore, toner images can be rapidly fixed on recording materials, and the thus fixed images have high glossiness and transparency. However, the contact heat fixing methods tend to cause an offset problem in that part of a fused toner image, which contacts the surface of a heating roller under pressure, is adhered and transferred to the surface of the heating roller, and then the part of the toner image is re-transferred to an undesired portion of the sheet itself or the following sheet of the recording material.
To prevent occurrence of the offset problem, the surface of the fixing roller typically includes materials which have good releasability such as silicone rubbers and fluorocarbon resins. Further, a release oil such as silicone oils is applied to the surface of the fixing roller. However, in this case, the fixing device needs an oil feeding device and therefore the image forming apparatus is upsized, i.e., a compact image forming apparatus cannot be provided. In attempting to solve these problems, the following techniques are used for toners for use in monochrome machines:    (1) raising viscoelasticity of the toner to prevent occurrence of the inner rupture when the toner is melted, by controlling the molecular weight of the binder resin included in the toner;    (2) a release agent is included in the toner.Such a toner can be well fixed by oilless fixing devices without a fixing oil applying system or fixing devices applying a small amount of oil, resulting in downsizing of the machine.
Recently, similarly to monochrome machines, full color machines have been also miniaturized and have an oilless fixing device. As mentioned above, the surface of a full color toner image must be smoothened to some extent to obtain a brilliant full color image having good color reproducibility, and therefore the viscoelasticity of full color toners must be decreased. Therefore, full color toners cause the offset problem more frequently than monochrome toners. Therefore, it is difficult to use the full color toner for oilless fixing devices without a fixing oil applying system or fixing devices applying a small amount of oil. In addition, a toner including a release agent has large adherability, and thereby transferability of the toner to a recording paper deteriorates. Moreover, the release agent tends to contaminate frictional charging members such as carrier, resulting in deterioration of chargeability and durability of the frictional charging members.
On the other hand, carriers having a cover layer are typically used, because of:    (1) preventing formation of a toner film on the surface of the carrier;    (2) uniformizing the surface of the carrier;    (5) lengthening the life of the developer;    (6) preventing adherence of the carrier to the surface of the photoreceptor (this phenomenon is hereinafter referred to as carrier adherence);    (7) protecting the photoredeptor from being abraded by the carrier;    (8) controlling charge polarity of the carrier (i.e., of the toner); and    (9) controlling charge quantity of the carrier (i.e., of the toner).
Published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 58-108548 discloses a carrier having a cover layer including a specific resin. JP-As54-155048, 57-40267, 58-108549, 59-166968 and 06-202381, and Published examined Japanese Patent Application Nos. 01-19584 and 03-628 have disclosed carriers having a cover layer including various specific additive agents. JP-A 05-273789 discloses a carrier to the surface of which an additive is adhered. JP-A 09-160304 discloses a carrier having a cover layer including electroconductive particles having the particle diameter larger than the thickness of the cover layer. JP-A 08-6307 discloses a carrier having a cover layer including a benzoguanamine-(n-butyl alcohol)-formaldehyde copolymer. Japanese patent No. 2683624 discloses a carrier having a cover layer including a crosslinking reaction product of a melamine resin with an acrylic resin.
However, these carriers do not have sufficient durability and resistance to carrier adherence. In particular, these carriers cause problems in that the toner mixed with the carriers has unstable charging property due to toner adherence to the carrier, and electric resistance of the carrier is decreased due to abrasion of the cover layer, namely the carriers have poor durability. Particularly, the qualities of initial images produced by developers including the carriers are good, but the image qualities gradually deteriorate as the number of produced printings increases.
Recently, image forming apparatus are required to produce high quality images at a high speed. Such a high-speed machine applies a large amount of mechanical stress to the developer, and thereby shortening the life of the developer. In addition, carbon black, which is typically included in the cover layer of a carrier as an electric resistance controlling agent, tends to leave therefrom due to abrasion of the cover layer, resulting in occurrence of a problem in that the released carbon black is mixed with color toners, resulting in deterioration of color tone of the produced color image (this problem is hereinafter referred to as a color tone changing problem).
In attempting to solve these problems, JP-A 07-140723 discloses a carrier without a cover layer, in which an electroconductive material (carbon black) is present on the surface of a core. JP-A 08-179570 discloses a carrier having a cover layer including carbon black, wherein the cover layer has a concentration gradient of carbon black, i.e., the concentration of carbon black is high near the interface of the cover layer and a core, while the concentration of carbon black is low near the surface of the cover layer. No carbon black exists on the surface of the cover layer. JP-A 08-286429 discloses a carrier having a double cover layer. An inner cover layer including carbon black is located on a core, and an outer cover layer including a white electroconductive material is located on the inner cover layer. However, these methods cannot sufficiently prevent occurrence of the color tone changing problem.
In order to prevent such a problem, it is preferable to exclude carbon black from the carrier. However, when carbon black is not included in a carrier, electric resistance and chargability of the carrier increases. Chargability of a carrier mainly depends on the cover material used. Since carbon black largely affects the chargability, it is difficult to replace carbon black with other materials using conventional technology.